[7.17] Add a highlighter unit test base class (#85719) (#87413)

The vast majority of our highlighter tests are integration or rest tests, which exercise
the full ES stack but take a long time to run and are difficult to debug. We have a few
unit tests but they are testing very low-level behaviour, and don't interact with the fetch
phase or hit contexts. This commit adds a new HighlighterTestCase base class with some
helper methods that should fill the gap between these two sets of tests. It includes a
method that takes a MapperService, ParsedDocument and SearchSourceBuilder, and then
runs the appropriate highlighter fetch subphase over the resulting hit.

Author: romseygeek

server/src/test/java/org/elasticsearch/search/fetch/subphase/highlight/CustomUnifiedHighlighterTests.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.search.fetch.subphase.highlight;
+
+import org.elasticsearch.index.mapper.MapperService;
+import org.elasticsearch.index.mapper.ParsedDocument;
+import org.elasticsearch.index.query.QueryBuilders;
+import org.elasticsearch.search.builder.SearchSourceBuilder;
+import org.elasticsearch.search.fetch.HighlighterTestCase;
+
+import java.io.IOException;
+import java.util.Map;
+
+public class CustomUnifiedHighlighterTests extends HighlighterTestCase {
+
+    public void testSimpleTermHighlighting() throws IOException {
+
+        MapperService mapperService = createMapperService(mapping(b -> {
+            b.startObject("field");
+            b.field("type", "text");
+            b.endObject();
+        }));
+
+        ParsedDocument doc = mapperService.documentMapper().parse(source(b -> b.field("field", "this is some text")));
+
+        SearchSourceBuilder search = new SearchSourceBuilder().query(QueryBuilders.termQuery("field", "some"))
+            .highlighter(new HighlightBuilder().field("field"));
+
+        Map<String, HighlightField> highlights = highlight(mapperService, doc, search);
+        assertHighlights(highlights, "field", "this is <em>some</em> text");
+    }
+
+}

test/framework/src/main/java/org/elasticsearch/index/mapper/MapperServiceTestCase.java

@@ -547,6 +547,10 @@ protected final void withAggregationContext(
     }
 
     protected SearchExecutionContext createSearchExecutionContext(MapperService mapperService) {
+        return createSearchExecutionContext(mapperService, null);
+    }
+
+    protected SearchExecutionContext createSearchExecutionContext(MapperService mapperService, IndexSearcher searcher) {
         final SimilarityService similarityService = new SimilarityService(
             mapperService.getIndexSettings(),
             null,
@@ -567,7 +571,7 @@ protected SearchExecutionContext createSearchExecutionContext(MapperService mapp
             xContentRegistry(),
             writableRegistry(),
             null,
-            null,
+            searcher,
             () -> nowInMillis,
             null,
             null,

test/framework/src/main/java/org/elasticsearch/search/fetch/HighlighterTestCase.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+package org.elasticsearch.search.fetch;
+
+import org.apache.lucene.search.IndexSearcher;
+import org.elasticsearch.common.text.Text;
+import org.elasticsearch.index.mapper.MapperService;
+import org.elasticsearch.index.mapper.MapperServiceTestCase;
+import org.elasticsearch.index.mapper.ParsedDocument;
+import org.elasticsearch.index.query.ParsedQuery;
+import org.elasticsearch.index.query.SearchExecutionContext;
+import org.elasticsearch.search.SearchHit;
+import org.elasticsearch.search.builder.SearchSourceBuilder;
+import org.elasticsearch.search.fetch.subphase.highlight.FastVectorHighlighter;
+import org.elasticsearch.search.fetch.subphase.highlight.HighlightField;
+import org.elasticsearch.search.fetch.subphase.highlight.HighlightPhase;
+import org.elasticsearch.search.fetch.subphase.highlight.Highlighter;
+import org.elasticsearch.search.fetch.subphase.highlight.PlainHighlighter;
+import org.elasticsearch.search.fetch.subphase.highlight.UnifiedHighlighter;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class HighlighterTestCase extends MapperServiceTestCase {
+
+    protected Map<String, Highlighter> getHighlighters() {
+        Map<String, Highlighter> highlighters = new HashMap<>();
+        highlighters.put("unified", new UnifiedHighlighter());
+        highlighters.put("fvh", new FastVectorHighlighter(getIndexSettings()));
+        highlighters.put("plain", new PlainHighlighter());
+        return highlighters;
+    }
+
+    /**
+     * Runs the highlight phase for a search over a specific document
+     * @param mapperService the Mappings to use for highlighting
+     * @param doc           a parsed document to highlight
+     * @param search        the search to highlight
+     */
+    protected final Map<String, HighlightField> highlight(MapperService mapperService, ParsedDocument doc, SearchSourceBuilder search)
+        throws IOException {
+        Map<String, HighlightField> highlights = new HashMap<>();
+        withLuceneIndex(mapperService, iw -> iw.addDocument(doc.rootDoc()), ir -> {
+            SearchExecutionContext context = createSearchExecutionContext(mapperService, new IndexSearcher(ir));
+            HighlightPhase highlightPhase = new HighlightPhase(getHighlighters());
+            FetchSubPhaseProcessor processor = highlightPhase.getProcessor(fetchContext(context, search));
+            FetchSubPhase.HitContext hitContext = new FetchSubPhase.HitContext(
+                new SearchHit(0, "id", new Text("_doc"), null, null),
+                ir.leaves().get(0),
+                0
+            );
+            processor.process(hitContext);
+            highlights.putAll(hitContext.hit().getHighlightFields());
+        });
+        return highlights;
+    }
+
+    /**
+     * Given a set of highlights, assert that any particular field has the expected fragments
+     */
+    protected static void assertHighlights(Map<String, HighlightField> highlights, String field, String... fragments) {
+        assertNotNull(highlights.get(field));
+        Set<String> actualFragments = Arrays.stream(highlights.get(field).getFragments()).map(Text::toString).collect(Collectors.toSet());
+        Set<String> expectedFragments = new HashSet<>(Arrays.asList(fragments));
+        assertEquals(expectedFragments, actualFragments);
+    }
+
+    private static FetchContext fetchContext(SearchExecutionContext context, SearchSourceBuilder search) throws IOException {
+        FetchContext fetchContext = mock(FetchContext.class);
+        when(fetchContext.highlight()).thenReturn(search.highlighter().build(context));
+        when(fetchContext.parsedQuery()).thenReturn(new ParsedQuery(search.query().toQuery(context)));
+        when(fetchContext.getSearchExecutionContext()).thenReturn(context);
+        return fetchContext;
+    }
+}